1. Field of the Invention
The present invention relates to a recordable (including write-once and rewritable) optical disc and an optical disc device for recording thereon, and more specifically to the same suitable for recording at a so-called tuple speed higher than a normal recording speed.
2. Description of the Related Art
Conventional recordable optical discs are often used for recording at a tuple speed higher than a normal recording speed. Double or treble-speed recording advantageously reduces recording time and improves user-friendliness. Resulting recent demand for higher-speed recording has already caused octuple or more-speed drives commercialized.
However, due to its threshold strength, an optical disc may suffer from fracture in its polycarbonate substrate under overly high-speed rotation during tuple-speed recording. In particular, an HD DVD (High-Definition Digital Versatile Disc; hereinafter referred to as “HD”), which is one of the next-generation DVDs, rotates at a linear velocity of 6.61 m/sec even during normal-speed recording, and therefore, undergoes considerably high disc rotational speed (rpm; hereinafter the same shall apply) during tuple-speed recording. More specifically, disc recording on inner areas causes higher disc rotational speeds, and therefore, even quadruple-speed recording will cause rotational speeds higher than 10,000 rpm on an inner area bounded by a certain radius. It should be noted that the rotational speed of 10,000 rpm, at present, gives a guide as to whether to consider the threshold strength of an optical disc.
Therefore, the HD recording at a quadruple or more speed requires recording on an inner area at an over-threshold rotational speed. However, recording at such a high rotational speed does not necessarily bring proper characteristics to the recorded signals, but may cause insufficient reliability of the recorded information. In addition, although disc manufacturers usually conduct a test to guarantee reliability of their discs, a proper test can hardly be conducted at such a high rotational speed over 10,000 rpm because of characteristics of an inspection device, disc strength and others. Therefore, disc manufacturers supposedly conduct a test for recording characteristics only in a middle and/or outer circumferential area which allows low rotational speeds. Even if extending the test to the inside of that area, the manufacturers may only do in an area which allows rotational speeds lower than 10,000 rpm to enable the test.
In the aforementioned case, in light of user-friendliness, tuple-speed recording is preferably conducted on as wide an area as possible, whereas the tuple-speed recording even in an inner area, which does not guarantee recording characteristics, will reduce reliability of the recorded information and may cause important information to be lost.
The range of the inner circumferential area of the disc that guarantees the signal characteristics of tuple-speed recording, as described above, strongly depends on testing methods and performance of testing devices of disc manufacturers, and therefore, depends on the manufacturers and their discs. Thus, tuple-speed recording in accordance with a range uniformly set by a disc drive as recordable at a tuple speed may be performed, for some discs, even on an area where recording characteristics under an instructed tuple-speed are not guaranteed, thereby impairing reliability of recorded information, but not performed, for other discs, even on an area recordable at the tuple speed, therefore detracting user-friendliness.